Pulley
 

My '72, 402 Chevy truck has the 3 groove pulley on the water pump and the crankshaft. I am not running AC right now. I have a 1 groove pulley for the water pump that was on a '72. When I put the one groove pulley on the water pump it touches the crankshaft pulley. My question is - when they used the one groove pulley on the water pump, was the crankshaft pulley smaller? Why are the 2 pulleys touching each other? Thanks in advance to any response.